The invention relates to the electromechanical transducer of a limiter switch for alternating current, with an E-shaped armature and an E- or C-shaped core as described, for instance, in the Swiss Pat. No. 552.278. Such transducers have two undesirable properties which influence each other, to wit the hum induced by the alternating current, which they emit in the activated state, and the tendency of stationary and mobile magnetic pieces to stick together due to residual magnetism. Two measures are known in the art to counteract these features. One is the use of short-circuit rings and the other a shaping of the magnetic pole surfaces which ensures that adequate air gaps remain in existence even during activation, in the region where core and armature touch each other.
It can be inferred, for instance, from the aforementioned patent, that the choice of the support points where core and armature rest on each other is very critical with regard to the hum behavior of the current limiter. For reasons of stability, the support points were hitherto always located on the outer legs of both E- and C-shaped iron pieces. In this case, the short-circuit rings mentioned above, which are made of a nonferromagnetic material having a good electrical conductivity such as hard copper or brass for instance, are also located in the region of the outer legs, and the ratio of the pole surface enclosed by the rings to the non-enclosed pole surface is chosen so as to obtain an optimal hum suppression.
In order to avoid the sticking together of both magnetic pieces, it is usual to ensure that in the activated state there remains an air gap in the region of the inner leg or legs in order to sufficiently reduce the residual magnetism. However, this allows a flexural vibration of the magnetic pieces, which are in mutual contact at the frontal surfaces of their outer legs, and will induce a residual hum which is practically impossible to suppress. Furthermore, the pole surfaces must be planned to a fairly high degree of precision, both on the outer legs where the surfaces rest on each other and in between where these surfaces define an air gap. Indeed, not only must this gap be shaped to a close tolerance, but the outer supporting surfaces must also be pairwise exactly coplanar. The manufacturing costs of such magnetic core pieces is correspondingly high. The goal of the present invention is to create a limiter switch for alternating current which can be produced economically and yet have excellent functioning characteristics.